An air bag has been quickly increasing in its installing rate in recent years as one of the safety parts for cars. The air bag is used for such a purpose that, in car collisions, a sensor perceives the shock, gas of high temperature and high pressure is generated from an inflator and the air bag is quickly developed by the gas whereby the collision of the bodies or, particularly, the heads of a driver and a passenger to steering wheel, windshield, door glass, etc. is prevented and protected. In recent years, air bags for cars have been developed in their practical use not only for a driver seat and a passenger seat but also for knee air bags, side air bags, curtain air bags, etc. and installment of a plurality of air bags is now becoming common.
As regions and numbers of the installed air bags are increasing, there has been an increasing demand for making the weight and the size of an air bag system lighter and more compact and each of the parts of the system has been designed with a target of making the weight lighter and the size more compact. In view of such a background, air bags have been investigated for a plan of using a base fabric made of the yarn of small fineness or for a plan of reducing the type of elastomer of a coating woven fabric and the coating amount.
For example, fineness of a filament used in a coated base fabric for air bags is becoming finer from 940 dtex to 470 dtex and, in recent years, there has been a change to a base fabric which uses the filaments having fineness of 350 dtex.
Also, an elastomer resin which is coated to a coated base fabric for air bags has been changed from chloroprene to silicone resin. Further, its coating amount has been changed from 90-120 g/m2 to 40-60 g/m2 and, in recent years, it has been reduced to an extent of 25-40 g/m2. Although considerable enhancement has been achieved by such a means in view of package ability, it is not still in a fully satisfactory level and there has been a demand for improved package ability and light weight by further reduction of the coating amount.
On the other hand, a coated woven fabric which is coated with synthetic rubber or resin such as silicone has been mostly used for side air bags, curtain air bags, knee air bags, etc. where an inner pressure keeping property is particularly demanded. When the resin amount is reduced to 20 g/m2 or less in the woven fabric as such, the resin amount positioned on the surface extremely decreases, breakage of the resin film is apt to happen and keeping the low air permeability to a high extent becomes difficult.
With regard to a coated base fabric for air bags where the coating amount of silicone resin is reduced, there is a disclosure for an air bag where elastomer is unbalancedly present in terms of a coating thickness ratio of not less than 3.0 between thread and thread of the woven fabric to 1.0 of woven fabric yarn area which constitutes the woven fabric (see Patent Document 1).
Although package ability is improved in the above air bag, breakage of the resin film is apt to happen under the state where the resin is unbalancedly present as mentioned above when a coating amount is adjusted to 20 g/m2 or less and it is difficult to fully satisfy the low air permeability particularly for an air bag where the inner pressure keeping property is demanded.
There is also a disclosure for a coated base fabric for air bags where not less than 90% of the outer circumstances of the cross sections of warp and weft positioned at the resin-coated surface of the synthetic fiber woven fabric are surrounded by said resin and the coating amount of the resin is not more than 20 g/m2 (see claim 2 of Patent Document 2). In this base fabric, although the adhesion of the base fabric to the resin is enhanced due to impregnation of the resin, the resin film positioned on the woven fabric surface is thin whereby breakage is apt to happen on the film and it is difficult to fully satisfy the low air permeability particularly for an air bag where the inner pressure keeping property is demanded.